Bumping this, might help clarify matters for several other GPGX users!

ekeeke replied to me about this, explaining the following regarding the new MAME sound engines:

*ASIC version corresponds to the chip model used in later Model 1 consoles and most Model 2 consoles. It features the better (not distorded and less noisy) DAC output and full address decoding of status register (which causes slower music in Hellfire since it was developed for the discrete chip version) compared to discrete chip version.

Enhanced version has same "enhanced" features as ASIC version + full resolution DAC (14-bit instead of 9-bit in real hardware versions) + zero busy flag time (busy flag in FM status is only emulated for real hardware chip versions). It corresponds to an "idealized" version of the chip, like it is emulated in other Genesis emulators.

The low pass filter is unrelated to the chip DAC and exists in all console models (it is implemented by analog circuitry outside of the chip). It should be enabled for accurate emulation of analog sound output (and disabled if you prefer the clear sound of pure digital emulation). Filtering ratio is configurable because it varies for each console models (also between model 1 consoles or between model 2 ones) so it's up to you to find a value that matches your own memory of accurate sound. Default value should correspond to Model 1 VA4 console.*